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FREQUENCY CONTROL

-- Bhanu Bhushan --< a3bhanu@gmail.com >

(August, 2011)

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What is frequency ?

How would you explain it to a layman ?

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Number of times the voltage reverses

in half a second.

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From where does it come ?

What governs the frequency ?

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Speed at which the turbine - generators

are running at that time.

3000 rpm = 50 rps = 50.0 Hz

2940 rpm = 49 rps = 49.0 Hz

( for 2 - pole generators )

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Is it same all over a system ?

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Yes.

But, why ?

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Because polarity of voltages produced

by all generators must be same at a

particular time, and must reverse together.

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Is the frequency constant ?

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No.

Why not ?

Why does frequency keep changing

all the time ?

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• Because total consumer load keeps changing from time to time, and

• Because total generation is not changing in step with the change in total load.

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In a single-machine system, MW sent out from the generator is decided by and is equal to the total consumer load at that time. Steam flow to the turbine is decided by the control valve opening. So, to stop frequency from going on increasing (decreasing), steam flow to the turbine has to be equalized with the MW, by reducing (increasing) the control valve opening. This is the job of the governor.

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Governors have a drooping characteristic. If droop setting is 5%, it means that the control valve would be closed from 100% to 0% position when turbine speed rises by 5%, which would happen when the system frequency rises by 2.5 Hz (in a 50 Hz system).

If frequency rises (falls) by 0.1 Hz, turbine-generator output would be reduced (increased) by 4% of MCR automatically.

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The basic functions of a governor on a prime mover

• To vary the power input to match the power output, and thereby contain the speed change.

• Additionally, in case of grid-connected generating units, to enable stable parallel operation by equitable sharing of system load changes.

• To prevent dangerous over-speeding in case of a load throw-off; it is an essential safety device. Must always be in operation.

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• Frequency changes (magnitude of fluctuations) are much higher when governors are not in normal operation.

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2020

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What happens if the frequency deviates from the rated value of 50.0 Hz ?

What are the adverse effects of high and

low frequency ?

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• Motors & driven equipment : over-loading / reduced output

• Transformers : --- / over-fluxing

• Capacitors : --- / effectiveness falls

• Reactors : effectiveness falls / ---

• Transmission lines : parameters change

• Steam turbines : resonance of LP blades

• Generating stations : --- / MW capability falls

• Power system : effect on voltage profile.

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Permissible band of frequency ?

• Obligation to consumers

• Requirement of power plants

• Requirement of system operation

• Electric clocks.

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• Indian Electricity Rules, 1956 : +/- 3%

• IEC for Steam turbines : + 1%, - 2%

• IEGC ?

• England :

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• How is the frequency stabilized and controlled ?

- Established practice in developed countries

- In India

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(G1+G2+G3) = (L1+L2+L3)

GI

L1

G2

L2x

REGION

y

G3

REGION

L3

x=L1-G1 y=L3-G3

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• Suppose L1 increases to L1’. Frequency would decline to a new level at which (G1’+G2’+G3’) = (L1’+L2’+L3’)

• Then x’ = L1’ – G1’; y’ = L3’ – G3’• x’ may not be = x; y’ may not be = y• Such fluctuations keep happening all the time. • Let us assume that L2 & L3 remain unchanged

when frequency declines.• Then (G1’+G2’+G3’-G1-G2-G3) = L1’-L1.

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• If all generating units are on FGMO with an equal droop, (G1’-G1)/G1 = (G2’-G2)/G2 =

(G3’-G3)/G3 = (L1’-L1)/(L1+L2+L3).• In Western countries, it is mandatory to

continuously regulate G1 such that x’’ = x within 10-15 minutes, assuming x was = interchange schedule for the State to begin with.

• This is possible only if (G1’’-G1) = (L1’-L1).• This requires that every constituent has adequate

spinning reserves and AFC. (Self-sufficiency)• No concept of region-wide merit-order.

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Steps in frequency control

• Change of total generation in the system to match the new total system load, through FGMO, without any time delay.

• Change the total generation in affected area to absorb the change in area load.

• Change the generation in affected area to restore merit-order. May be combined with the previous step.

• Additional sale / purchase for economy.

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• Frequency and all tie-line flows automatically return to the previous level, without any corrective action in other control areas. The latter’s generation also returns to previous level, without any change in governor settings.

• Frequency = 50.0 Hz and x = schedule, to start with. Frequency = 50.0 Hz and x’’ = schedule, after the third step.

• Can this conventional scheme work after vertical unbundling of power utilities? How would a Discom keep to its import schedule?

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In our case, x generally is above / below schedule, to start with, and frequency may not = 50.0 Hz.

• Each State has notified entitlements in Central generating stations

• Suppose it is ‘a’ MW on a day• Then, (x – a) = over-drawal, for which the State

has to pay UI charges into the regional pool account. ‘x’ is metered in 15 minute wise energy.

• UI rate is determined for each 15-minute period according to average frequency during the period.

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• Each State system is a ‘notional’ control area. Schedules serve as commercial and operational datum, and deviations are allowed as long as system is not endangered and UI charges are paid.

• Concept of frequency-linked load dispatch, to achieve system-wide merit-order.

• No need for Steps 2 & 3. Step 4 is optional.• Governor settings are required to be adjusted in

all control areas, to bring the generating units back to desired MW.

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When frequency is low (because of a general deficit), UI rate would be high and the State should plan to curtail its over-drawal, by

-- Harnessing all generation within the State

-- Procuring additional power.

Load - shedding only when no additional power is available at a price that the State utilities can pay, and UI rate is not affordable or frequency has come down to a level which endangers the grid.